The present invention relates to a method and apparatus for judging road surface conditions and to a program for setting a threshold for judging road surface conditions. More particularly, it relates to a method and apparatus for judging road surface conditions and to a program for setting a threshold for judging road surface conditions with which it is possible to identify a tire that is presently mounted when a tire has been exchanged or other occasions and to set a threshold for determining a tendency of slip.
An apparatus for estimating coefficients of road surface friction xcexc (road surface xcexc) is known, for instance, from Japanese Unexamined Patent Publication No. 112659/1995. The road surface xcexc here is for indicating degrees of frictional bond between road surfaces and tires, wherein the larger this road surface xcexc is, the larger the acceleration generated by the vehicle will be and wherein the road surface xcexc is further characterized in that it increases in accordance with a rise in slip rate of the wheel in the range of approximately 10 to 20%. Based on the fact that the larger the slip rate of the vehicle is, the smaller the friction coefficient with the road surface will be even when the same vehicle acceleration is generated, the publication is considering a characteristic curve of vehicle acceleration-slip rate of the wheels S upon replacing the road surface xcexc with vehicle acceleration in the characteristic curve between the road surface xcexc and the slip rate of the wheels.
In the apparatus disclosed in the publication, the road surface xcexc is estimated on the basis of a relationship between a sum total of slip rates of respective wheels that have been summed up over a plurality of periods and the vehicle acceleration that has been summed up over a plurality of periods to thereby reduce influences of error factors such as noise. More particularly, the relationship between the slip rate and the vehicle acceleration is given as a ratio (gradient) M of vehicle acceleration xcexa3Ab integrated by a specified period of time to a sum of slip rates of the wheels xcexa3St integrated by a specified period of time (M=xcexa3Ab/xcexa3St), and road surface conditions are estimated on the basis of the value of M.
However, since the method of the Japanese Unexamined Patent Publication No. 112659/1995 requires it to preliminarily obtain the ratios M representing the relationship between the vehicle acceleration and the slip rate while utilizing values for coefficients of surface friction, which were obtained when the vehicle was made to run on a road surface of apparently high road surface xcexc, as references, setting needs to be performed for each of the tires. The values of M are also changed accompanying wear in tires or hardening of aging rubber.
It is therefore being wanted for a method with which it is possible to automatically set a threshold in accordance with presently mounted tires for judging road surface conditions.
The present invention has been made in view of the above facts, and it is an object thereof to provide a method and apparatus for judging road surface conditions that is capable of accurately setting a threshold for determining a tendency to slip upon identifying presently mounted tires, and a program for setting a threshold for judging road surface conditions.
According to a first aspect of the present invention, there is provided a method for judging road surface conditions, the method including the steps of: periodically detecting rotational information of tires of four wheels of a vehicle; storing the rotational information of the respective tires; calculating a vehicle velocity from the rotational information; calculating an acceleration/deceleration of the vehicle; calculating a slip ratio of left-sided front and rear wheels and right-sided front and rear wheels and a slip ratio of front and rear wheels, respectively; calculating an amount of fluctuation in differences in slip ratios between the right and left wheels; obtaining a linear regression coefficient and a correlation coefficient between the slip ratio of the front and rear wheels and the acceleration/deceleration of the vehicle; and setting a threshold for judging road surface conditions on the basis of the linear regression coefficient if the amount of fluctuation is a specified value.
According to a second aspect of the present invention, there is provided an apparatus for judging road surface conditions including a rotational information detecting means for periodically detecting rotational information of tires of four wheels of a vehicle; a rotational information storing means for storing the rotational information of the respective tires; a vehicle velocity calculating means for calculating a vehicle velocity from the rotational information; an acceleration/deceleration calculating means for calculating an acceleration/deceleration of the vehicle; a slip ratio calculating means for calculating a slip ratio of left-sided front and rear wheels and right-sided front and rear wheels and a slip ratio of front and rear wheels, respectively; a fluctuation amount calculating means for calculating an amount of fluctuation in differences between the right and left slip ratios; a coefficient calculating means for obtaining a linear regression coefficient and a correlation coefficient between the slip ratio of the front and rear wheels and the acceleration/deceleration of the vehicle; and a threshold setting means for setting a threshold for judging road surface conditions on the basis of the linear regression coefficient if the amount of fluctuation is a specified value.
According to a third aspect of the present invention, there is provided a program for setting a threshold for judging road surface conditions, wherein a computer is made to function as a rotational information storing means for storing rotational information of respective tires; a vehicle velocity calculating means for calculating a vehicle velocity from the rotational information; an acceleration/deceleration calculating means for calculating an acceleration/deceleration of the vehicle; a slip ratio calculating means for calculating a slip ratio of left-sided front and rear wheels and right-sided front and rear wheels and a slip ratio of front and rear wheels, respectively; a fluctuation amount calculating means for calculating an amount of fluctuation in differences between right and left slip rates; a coefficient calculating means for obtaining a linear regression coefficient and a correlation coefficient between the slip ratio of the front and rear wheels and the acceleration/deceleration of the vehicle; and a threshold setting means for setting a threshold for judging road surface conditions on the basis of the linear regression coefficient if the amount of fluctuation is a specified value.